1. Field of the Invention
The invention relates to a device for tire pressure surveillance with a pressure measurement member furnishing a tire pressure depending signal and an evaluation unit actuated upon a falling of the tire pressure below a tire pressure limiting value and actuated by the signal of the pressure measurement member as well as a correction member correcting the signal of the pressure measurement member depending on temperature.
2. Brief Description of the Background of the Invention including Prior Art
Such a device for surveillance of tire pressures is known from the European Patent EP-0 248 821 B. The device according to the European Patent EP-0 248 821 B is disposed following to the tire interior in the rim base. The pressure measurement member and the correction member for correcting the signal of the pressure measurement member depending on temperature are united and integrated constructively. This results of the formation of the pressure measurement member as a pressure measurement box, which furnishes a pressure depending stroke as a signal. The known device employs as an evaluation unit a magnetic distance path sensor capturing the stroke furnished by the pressure measurement box as a signal of the pressure measurement member. The known device is based on the idea that the included reference volume in the pressure measurement member assumes the tire air temperature based on a direct contact between the tire air and the pressure measurement member and that the pressure of the reference volume and thus also the signal of the pressure measurement member are changing with this temperature of the tire. Thus, it is a purpose to exclude errors in the tire pressure limiting value caused by the air temperature in the tire.
The temperature and thus the pressure of the reference volume do not depend only on the tire air temperature but also on the heat stream coming from the base rim into the device. This heat stream and the temperature increase caused in the device by this heat stream can be substantial, since the rim and in particular the rim base are heated up very much, based on the braking device incorporated into the rim and based on actuation of the brake. It has been shown that the signal of the pressure measurement member is thus overcompensated based on this heat flow and temperature flow, that the evaluation unit is erroneously actuated. Depending on the kind of the use made of the actuation of the evaluation unit, false and incorrect tire pressure information and/or an erroneous tire pressure automatic control process are initiated and employed.
In order to avoid this disadvantage it is known to insulate the device for tire pressure surveillance by intermediately disposing heat insulating material and by using similar measures against the heat stream from the base of the rim. However, these steps are associated with substantial expenditures.
The German Patent Application Laid Open DE 35 39 89 to Manfred Gerresheim et al. teaches a method for determining a variable air pressure value of a vehicle air tire and to display of such pressure value. The Figs. of the reference show in the diagram the tire overpressure and ploted against the inner temperature. Methods are indicated to determine the pressure deviation depending on the set point tolerance regions and a certain pressure state and to display an air pressure state taking into consideration regional and seasonal caused pressure and temperature influences. Apparently, heat and temperatures caused by braking are not compensated.
The German Patent Application Laid Open DE- 34 45 854 to Erich Zabler teaches a device for surveillance of predetermined values in motor vehicle tires. In particular a sensor unit is disposed at the tire wheel which transfers an emitted signal which depends on the pressure in the tire to a fixed position receiver (14). The sensor allows to survey both the tire pressure as well as the tire temperature. An influence of heat flow to the rim from braking is not considered in the reference.
The German Patent Application Laid Open DE 28 54 518 to Gunter Pruss teaches a device for surveillance and display of the pressure in a motor vehicle tire. The device substantially includes a shaft (1) and a membrane receiver (2) which are solidly connected to each other. Both the shaft (1) and the membrane receiver (2) of the reference are formed as hollow cylinders. The membrane (5) then can perform motions in the direction of the longitudinal axis of the shaft. The inner pressure of the tire is transferred to the membrane (5). It is submitted that this reference system does not lead to an automatic correction of the physical and environmental parameters to which the tire or base rim are subjected.
The U.S. Pat. No. 4,384,543 to Jacob Wong teaches a combined underinflation indicator and relief valve. The reference shows bellows 18 which are sealed to the atmosphere and this causes these bellows to be sensitive to a surrounding pressure change. The device operates such that the tire maintains its full pressure regardless of tire temperature. The hot air inside of the tire will heat up the pressure responsive housing 16 and heat up the bellows 18 causing it to tend to expand and thereby counteract the air pressure inside the tire. The bellows 18 of the reference acts as temperature sensing actuator for a relief valve.